This invention relates to a printing method and system whereby the information printed on a substrate, such as paper, is transferred in the form of a latent image or "secure image" which is invisible to the eye and any other usual image detecting device at the time of printing and is revealed only after the substrate is subjected to a subsequent process of image activation.
Invisible inks have existed for many years. Many such ink systems have been found, developed and used in a limited way mainly because of the limited accessibility of such invisible inks and delivery systems for the latter.
A problem with the use of latent image printing for the purpose of product identification was that no good means existed for placing such information on a variety of substrates. Contact printing is not suitable for many such uses as there is no good means to place the image on the desired substrate if it is irregularly shaped or is substantially inaccessible to traditional contact printing devices.
Thus, latent image, variable information such as production date, lot number, batch number, serial number, and the like, could not be placed on many products, except by hand, which is cumbersome, expensive, and prone to inaccuracies, defeating the purpose of using such information.
To address such problems, U.S. Pat. No. 5,395,432 taught the use of certain ink jet compositions for use in printing latent images on a substrate. In accordance with that patent, ink jet compositions comprising zinc chloride were taught for use in making latent images. A problem associated with that formulation, however, was its corrosiveness to metal parts of the printing apparatus with which the ink comes into contact. A need therefore arose for an ink jet composition that met the goals of the composition taught in U.S. Pat. No. 5,395,432, while providing reduced corrosiveness.
Further, such an ink jet composition must meet certain rigid requirements to be useful in ink jet printing operations. These relate to viscosity, resistivity, solubility, compatibility of components and wettability of the substrate. Also, the ink must be quick-drying and smear resistant, must be capable of passing through the ink jet nozzle without clogging, and must permit rapid cleanup of the machine components with minimum effort.